This invention relates to a process for preparing high absorbtivity sodium tripolyphosphate (STPP) particles by a simple modification of existing processing equipment which is currently used commercially to manufacture sodium tripolyphosphate.
Sodium tripolyphosphate is widely used in the formulation of modern detergent compositions where it acts as a phosphate builder, increasing the cleaning ability of the detergent composition.
A key property of sodium tripolyphosphate in this application is its ability to absorb various liquid components used in the formulation of detergents, such as surfactants. Absorbtivity is the measure of the ability of the sodium tripolyphosphate to soak up liquids without getting wet, and is expressed as the weight percent of Triton.RTM. X-100 which can be absorbed by the material without losing its free flowing properties. High absorbtivity values are beneficial for formulating free-flowing detergent compositions containing liquid surfactants, such as dry-mixed automatic dishwashing compositions. Such high absorbtivities allow the detergent compositions to incorporate greater amounts of surfactants without lumping or caking. Sodium tripolyphosphate compositions having this characteristic impart greater cleaning power to detergents since they are capable of absorbing and retaining large quantities of surfactants without forming lumps during the agglomeration or dry-blending manufacturing operation.
Typical grades of commercial sodium tripolyphosphates generally have absorbtivity values of between about 6% and 20%, depending on the density of the material, with the absorbtivity generally increasing as the density is decreased. Although absorbtivity values in this range are satisfactory for many applications, the performance of modern detergent compositions is continually being upgraded, and consequently, it would be desirable to increase the absorbtivity even further to enhance the performance of these materials.
Sodium tripolyphosphate is produced commercially from an aqueous solution of monosodium phosphate and disodium phosphate, which can be prepared by reacting phosphoric acid with sodium hydroxide or sodium carbonate. This "wet mix" is typically spray-dried to produce granules of dehydrated sodium orthophosphate. The sodium orthophosphate is then converted to sodium tripolyphosphate by calcining the particles at elevated temperatures of from 300.degree. C. to 600.degree. C.
Various techniques have been proposed for increasing the absorbtivity of sodium tripolyphosphate. Two such techniques are disclosed in U.S. Pat. Nos. 4,251,498 and 4,255,274, both to Hensler et al. In U.S. Pat. No. 4,255,274, a dense sodium tripolyphosphate particulate mixture having a bulk density of at least 56 lbs./ft..sup.3 is moisturized by the addition of 10% by weight moisture to the particles, and these particles are then calcined at elevated temperatures to produce STPP particles having absorbtivities of up to 22.4%. U.S. Pat. No. 4,251,498 is an application of the previous process to undersized STPP particles, which are first compacted and milled prior to moisturization and calcining. This process is effective to produce an STPP product having an absorbtivity of up to 22.1%.
As will be appreciated by those skilled in the art, it would be desirable to increase the absorbtivity of sodium tripolyphosphate to even higher levels than achievable using existing technology using a relatively inexpensive modification to existing processing equipment used to manufacture commercial STPP.